1. Field of the Invention
The present invention relates generally to a mobile communication system that sends packet data on an uplink, and more particularly, to a method and apparatus for enabling a User Equipment (UE) to efficiently implement a packet service by non-scheduled transmission.
2. Description of the Related Art
In a Wideband Code Division Multiple Access (WCDMA) communication system, a new channel called Enhanced Uplink Dedicated Channel (E-DCH or EUDCH) was introduced to improve the performance of uplink packet transmission.
The E-DCH mobile communication system has been enhanced with Node B-controlled scheduling and Hybrid Automatic Repeat reQuest (HARQ) to provide maximized uplink transmission efficiency. HARQ between a UE and a Node B increases a transmission success rate for a given transmission power. In HARQ, the Node B soft-combines an erroneous data block with its retransmission block, instead of discarding the erroneous data block, thereby increasing the reception success probability of the data block.
FIG. 1 illustrates an overview of Node B-controlled scheduling in the E-DCH communication system.
Referring to FIG. 1, a Node B 105 receives from UEs 110 and 115 reports of status changes and buffer occupancies of the UEs 110 and 115 by Scheduling Information (SI) 120. The Node B 105 controls uplink transmissions from the UEs 110 and 115 based on the SI 120. For the purpose of efficient use of limited uplink resources, the Node B 105 sends a grant 125 permitting transmission of a large amount of data to the UE 110 in a good channel status, whereas it sends a grant 135 permitting transmission of a small amount of data to the UE 115 in a poor channel status. The UEs 110 and 115 send uplink packets 130 and 140 within their allowed data amounts.
Due to the fact that the UE reports its buffer occupancy and then receives a grant representing a scheduling result from the Node B, the above Node B-controlled scheduling scheme is not viable for a delay-sensitive service with small-size packets that are continuously generated. For instance, Voice over Internet Protocol (VoIP) is sensitive to delay and preferably, VoIP packets are sent immediately when they are created. With the buffer occupancy report and the scheduling, the VoIP packet transmission will suffer a long time delay.
One of the features of a service such as VoIP is a relatively constant packet size. To efficiently support the delay-sensitive service with continuously generated packets of a relatively constant small size, the concept of non-scheduled transmission has been introduced. The non-scheduled transmission scheme allows the terminal to autonomously send packets of up to a predetermined size without grants from the Node B in a service flow allowing non-scheduled transmission.
The Node B determines whether to allow the UE non-scheduled transmission, taking into account the characteristics or Quality of Service (QoS) requirement of a UE-requested service. Non-scheduled transmission is allowed for the UE on a service flow-by-service flow basis. In a Universal Mobile Telecommunication System (UMTS) communication system, service flows with the same QoS are collectively called a Medium Access Control (MAC)-d flow and non-scheduled transmission is set on a MAC-d flow basis.
A parameter defining non-scheduled transmission is a maximum MAC-e Protocol Data Unit (PDU) content size, i.e. a maximum packet size allowed for non-scheduled transmission. The terminal can send packets of up to the maximum MAC-e PDU content size, generated in a MAC-d flow allowing non-scheduled transmission, without grants from the Node B.
FIG. 2 is a diagram illustrating a signal flow for a typical non-scheduled transmission.
Referring to FIG. 2, a Radio Network Controller (RNC) 215, which is responsible for managing radio resources between a Node B 210 and a UE 205, signals the Identifier (ID) of the MAC-d flow and the maximum MAC-e PDU content size to the Node B 210 and the UE 205 in step 220. The UE 205 can autonomously send packets at or below the maximum MAC-e PDU content size, which are generated in the MAC-d flow, without grants from the Node B 210 in step 225. Considering the fact that the packets at or below the maximum MAC-e PDU content size can be received in the MAC-d flow at any time, the Node B 210 schedules other UEs and other service flows.
However, the above non-scheduled transmission scheme may cause a degradation in the quality of VoIP, which is against the original intention of the non-scheduled transmission scheme. In an early stage of VoIP communications, a few relatively large packets are generated. Unless a maximum MAC-e PDU content size set for the VoIP service is large enough to enable transmission of one large packet at one time, this packet is sent separately a plurality of times. On the contrary, if the maximum MAC-e PDU content size is large enough for the initial packets, resource dissipation results after small packets are generated. That is why the conventional non-scheduled transmission scheme inevitably suffers from voice quality degradation and delay in an early communication stage.